The present invention relates to the field of intravascular radiation therapy. In particular, the present invention relates to catheters used for intravascular delivery of radiation.
Coronary artery balloon angioplasty is a minimally invasive technique developed as an alternative to coronary artery bypass grafting for treatment of atherosclerosis, the principle process of heart disease. There are about 450,000 coronary interventions, i.e., angioplasty, atherectomy, and stent, performed annually in the U.S. However, a major limitation of this clinical procedure is the high prevalence of restenosis, or re-narrowing, of the treated vessel. Restenosis occurs approximately 30-50% of the time.
Restenosis occurs as a result of injury to the vessel wall due to the angioplasty procedure, or to other procedures, i.e., stenting, atherectomy, that compress or remove the atherosclerosis and may cause injury to the vessel. Restenosis is a complex process, which can involve immediate vascular recoil, neointimal hyperplasia, and/or late vascular remodeling. Neointimal hyperplasia, a response of the body to balloon-induced physical injury of the vessel wall, is thought to be the main contributor to restenosis. Hyperplasia can result in narrowing of the vessel lumen within 3-6 months after angioplasty due to proliferation of smooth muscle cells in the region injured by the angioplasty. Restenosis can require the patient to undergo repeat angioplasty procedures or by-pass surgery with added costs and risks to the patient.
One method currently used to inhibit restenosis following a procedure such as angioplasty, involves delivery of a prescribed dose of radiation to the walls of the dilated length of vessel through intravascular radiotherapy (IRT). In an example of one method of IRT, a catheter is inserted into a vessel and positioned within the length of vessel dilated by the angioplasty procedure. Once the catheter is positioned, a radiation source is inserted into the lumen of the catheter and positioned to allow delivery of a prescribed dose of radiation to the vessel. Typically the prescribed dose of radiation is delivered at a dose level necessary to inhibit restenosis, and may be termed a therapeutic dose.
Some catheters used in IRT (and some radiation sources where a catheter is not used) may have a smaller diameter than the diameter of the vessel lumen. This differential in diameters provides space for the catheter to move radially as it is positioned within the vessel. If this differential is large enough, a catheter may move and become flexed within the vessel so that the radiation source is not centered within the vessel lumen. In some cases, portions of the catheter may be close to one side of the vessel wall and far from the opposite side and can result in delivery of a non-uniform dose of radiation to the vessel.
FIG. 1 illustrates a longitudinal cross-sectional view of one example of a catheter containing a radiation source inserted within a vessel in the prior art. In the illustration, a catheter 10 is inserted into a vessel 12. The catheter 10 has a longitudinal lumen 14 to receive a radiation source 16. As shown, the catheter 10 has flexed within the vessel during positioning so that portions of the radiation source 16 are not radially centered within the vessel. This positioning can result in the radiation source 16 being closer to the vessel wall at some points (shown at pointer A) and farther from the vessel wall at other points (shown at pointer B).
For a given radiation source, the dose rate drops rapidly as a function of distance from the source axis. Thus, a small change in distance from the source to the surface of the vessel wall can result in a large difference in the radiation dose received by the vessel. If the radiation source 16 is positioned close to the vessel wall, the vessel may receive an overdose of radiation, e.g., a hot spot. Overdosing a vessel wall with radiation can result in vessel damage, such as inflammation, hemorrhaging, and arterial necrosis. Conversely, the side of the vessel wall where the radiation source 16 is positioned away may receive an underdose of radiation so that restenosis may not be inhibited.
In order to mitigate overdosing or underdosing of a vessel, other catheters, such as centering catheters, have been developed to provide more stability in the radial positioning of the radiation source. By positioning the radiation source so that it is substantially centered within the vessel, a more uniform dose of radiation is delivered to the vessel. U.S. Pat. No. 5,643,171 to Bradshaw et al. describes several embodiments of a centering catheter that may be used with IRT. In one embodiment, a centering segment, such as a centering balloon, is attached to the portion of the catheter in which the radiation source is to be located. The balloon can then be inflated so that the radioactive source is substantially centered within the lumen of the vessel.
Recent results from clinical trials and animal studies, however, show a problem in a few of the cases following IRT, in which restenosis occurs at a higher incidence at one or both ends of the treated dilated length of the vessel, as opposed to the middle of the treated length of the vessel. This problem, known as the xe2x80x9cedge effectxe2x80x9d, is common with most current IRT systems, and can subject the patient to the same risks and costs that were initially sought to be avoided by using IRT.
The occurrence of edge effects suggests that an additional length of vessel to each side of the dilated length of vessel may also be injured by stretching and tears to the intima from the angioplasty, or other procedures, and possibly from the radiotherapy catheter. As the radiation source is typically chosen to treat the dilated length, the additional length of injured vessel may not receive a therapeutic dose of radiation.
Further, with some radiation sources used in IRT, the radiation dose delivered at the ends of the radiation source may be less than the dose delivered in the middle. FIG. 2 illustrates the longitudinal dose profile of a radiation source within a centering catheter in the prior art. Typically, a centering catheter 22 is positioned in a vessel so that the centering segment 24 is located within the length of the vessel injured during an intravascular procedure, i.e., the dilated length. For example, if a vessel was dilated with a 27 mm angioplasty balloon, a centering catheter 22 may have a 27 mm centering segment 24 within which a radiation source 26 will be positioned. The dose profile illustrates that if the 27 mm radiation source 26, such as a P32 radiation source, is positioned correctly within the dilated length, the 27 mm radiation source 26 delivers a therapeutic dose of radiation along a length of about 22 mm with a 2-2.5 mm dose fall off at each end. Thus, the radiation source 26 delivers a therapeutic dose of radiation along a length that is shorter than the dilated length of the vessel. This leaves little to no margin for treating additional lengths of injured vessel beyond the dilated length and does not allow room for positioning errors arising from the radiation delivery system or physician.
However, to attempt to treat this additional length of injured vessel by simply using a longer radiation source within the catheter is problematic. Current centering catheters typically do not extend the radiation source beyond the ends of the centering segment, and, typically, do not provide centering for radiation sources that extend beyond the ends of the centering segment. In these catheters, if a radiation source is extended beyond the centering segment in an attempt to treat the additional length of injured vessel, it could potentially damage the vessel by overdosing the vessel wall.
Extending both the length of the centering catheter and the radiation source within it doesn""t provide an optimal solution as a longer centering catheter may result in further damage to the vessel compounding the problem. Injury from the centering segment may not be as damaging as the angioplasty balloon but may result in restenosis which is ultimately what the IRT was attempting to avert.
Thus, what is needed is an apparatus that substantially centers a portion of a radiation source within a vessel along a desired treatment length to allow delivery of an approximately uniform dose of radiation and prevents overdosing of the vessel from portions of the radiation source that extend beyond the centered length. Further, the apparatus should mitigate additional injury to the vessel walls outside the centered length.
The present invention includes a stepped centering catheter for delivery of intravascular radiation therapy that substantially centers a portion of a radioactive source within the lumen of a vessel along a therapeutic treatment length and offsets portions of the radiation source that extend beyond the therapeutic treatment length within a region having a minimum offset from the vessel wall.